| Summary: | Immunoglobulin E (IgE) is fundamental to the allergic response and the functions of IgE are mediated by its Fc region binding to two receptors, FcεRI and CD23 (FcεRII). The interaction of IgE with other proteins have complicated our investigations of the unique role each receptor plays. To solve this, a small-scale library of IgE-Fc proteins was designed with two key positions, one at each receptor-binding site mutated. The unpredictable allosteric nature of IgE prevents rational engineering approaches, thus the design of a membrane-bound IgE-Fc-GFP-tagged protein allowed for the generation of a membrane-surface display library of stable cell lines. A FACS selection assay identified IgE-Fc proteins with weakened binding to a single IgE-receptor, which serves as a proof-of-principle for this concept. Additional studies into human CD23 and the differences between it and murine CD23 revealed additional levels of regulation for IgE-binding not seen in other species and this is due to its unique properties. Human CD23 is an unusual antibody receptor, being a calcium dependent (C-type) lectin that has lost its carbohydrate binding capability. Ca2+ binds to and increases CD23’s affinity for IgE, and one of two Ca2+ binding sites usually present in C-type lectins is absent in human but present in murine CD23. To understand if the loss of the second Ca2+ binding site has led to a regulatory gain/loss of function in human CD23, a panel of CD23 mutant proteins with increasingly ‘mouse-like’ sequences was generated. The insertion of the second Ca2+ binding site was verified by HSQC-NMR whilst molecular dynamic simulations provided a means of understanding the flexibility of the proteins. It revealed that binding of two Ca2+ ions tethers the soluble CD23 loops into position in the most mouse-like mutant protein, limiting possible conformations for IgE binding. Complementary Biacore experiments indicated that higher calcium binding affinity may have come at a cost of weakened IgE binding, as data in the presence and absence of Ca2+ showed decreased binding affinities of the proteins for human IgE. This regulatory difference between murine and human soluble CD23 could inform the development of CD23/IgE inhibitor therapeutics for the treatment of allergy.
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